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A dead fish could cause an ammonia reading. You might want to check your ammonia.
if it shows anything other than 0 I'd start looking in those hidey holes for a dead fish. if it's been several weeks, however, and the fish died, it may have mostly decomposed.
And yes - you should see both of them from time to time. even if one is being picked on, it should come out occasionally. Did you ever see aggression between them before the one disappeared?
Good luck. hope it's not bad news.

Well, it's a good sign that the ammonia is okay but still, one has to wonder
When you do your next water change take it down to below 50% then check inside the caves. all you have to do is lift carefully and slowly then replace them in the same spot. Only your cats and the GBr's are using the caves and that shouldn't stress them out over much if you don't rearrange anything.
will be eager to know what you find.
Is there any chance it could have gotten sucked into your filter intake tube?

I concur with fishmommie. I am only going to add something, thinking ahead here. If you have lost one ram, be careful getting a replacement as this may not work.

Mikrogeophagus ramirezi (common or blue ram) and the sister species M. altispinosus (Bolivian Ram) are fish that form bonded pairs (male/female) but they have to select their mate. Even in a 4-foot 55g tank, weaker fish can be harassed to the point of death by a male. I have a male Bolivian in my 5-foot tank, and found him a female a couple of years ago, and even though they spawned four times, they never really bonded and he killed her simply by hounding her before I realized it.

You might want to keep just the one remaining ram, or perhaps the store would allow you to return it if you buy a pair. You can usually see bonded pairs in the tank; I will explain if asked. Alternatively, have several in the tank can work, maybe 6-7, but there can still be problems down the road as pairs will form and spawn, and other rams are not always tolerated in the same space. It is not just the physical aspect, creating territories and such; fish secrete pheromones which other fish in the species read, like chemical signals, and these can be highly stressful.

Byron.

Byron Hosking, BMus, MA
Vancouver, BC, Canada

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]